This invention relates to laser trimmable coatings for use in laser trimming of film resistors, methods of applying such coatings and to film resistor devices employing such coating.
During the production of film resistors which are suitable for high speed laser trimming it is highly desirable to protect the resistors particularly from abrasion during processing of the resistors. In order to accomplish this, laser trimmable protective coatings are frequently applied to the resistor. These coatings which are generally heat hardenable but which may be hardenable by use of ultraviolet or other types of radiation including light in the past have been formed of certain such materials as thermosetting phenolic resins and glass glazes. However the use of glass glazes has been found to provide inadequate abrasion protection resulting in decreased yields and to be costly. The phenolic coating also has been found to be undesirable as the laser trimming through such coating results in decreased resistability due to coating carbonization.
An object of this invention is to provide a new and novel laser trimmable coating composition for film resistors through which laser trimming may readily take place and which provides increased abrasive resistance for the resistor. An additional object of this invention is to provide a laser trimmable coating for film resistors that would permit of a considerable decrease in the cost of production of the resistors.
These and other objects of the invention will be apparent from the description that follows.
In the foregoing disclosure of the invention the term "laser trimming" refers to a method of adjusting the electrical resistance of a resistor by laser removal of resistor material until the desired resistance value is achieved. Such a technique is described for example in the E. I. Du Pont de Nemours and Company publication No. A-99397 (7/74) "Laser Trimming Techniques for Thick Film Resistors".